that is the right way to start, although it's sin^2(x) = (1-cos^2(2x))/2, if we apply that to our integral, we get:
\[(1/2)\int\limits_{}^{}(1- \cos^2(18x))\].
The integral of 1 is simple, but be sure to watch out for the 18x inside the cosine:
\[= (x - [\sin(18x)/18])/2 + C\] (don't forget +C )